Recognize a closure struct of a generic method or type

FindRefStructParameters dropped generic instantiations, so a parameter
typed 'ref <>c__DisplayClass0_0<T>' never reached RefStructTypes. Both
consumers therefore missed local functions whose declaring type or method
is generic: the signature test for an obfuscated local function, and
LocalFunctionNeedsAccessibilityChange, which left such a function internal
while its closure struct stayed private - the recompiled output then fails
with CS0051.

Cross-module signatures still drop out, because the generic type part of an
instantiation goes through GetTypeFromReference, which returns nil.

Assisted-by: Claude:claude-opus-5[1m]:Claude Code

ICSharpCode.Decompiler/IL/Transforms/LocalFunctionDecompiler.cs

@@ -873,7 +873,10 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 
 			public TypeDefinitionHandle GetArrayType(TypeDefinitionHandle elementType, ArrayShape shape) => default;
 			public TypeDefinitionHandle GetFunctionPointerType(MethodSignature<TypeDefinitionHandle> signature) => default;
-			public TypeDefinitionHandle GetGenericInstantiation(TypeDefinitionHandle genericType, ImmutableArray<TypeDefinitionHandle> typeArguments) => default;
+			// A closure struct of a generic method or generic declaring type arrives as an instantiation;
+			// its definition handle is what identifies the struct. Cross-module generic types still drop out,
+			// because GetTypeFromReference already returned nil for them.
+			public TypeDefinitionHandle GetGenericInstantiation(TypeDefinitionHandle genericType, ImmutableArray<TypeDefinitionHandle> typeArguments) => genericType;
 			public TypeDefinitionHandle GetGenericMethodParameter(Unit genericContext, int index) => default;
 			public TypeDefinitionHandle GetGenericTypeParameter(Unit genericContext, int index) => default;
 			public TypeDefinitionHandle GetModifiedType(TypeDefinitionHandle modifier, TypeDefinitionHandle unmodifiedType, bool isRequired) => default;